I moved two vertices on the cube: once side is bent inwards and the other not
To add to the other answer, it is possible to fix errors in the automatic triangulation of meshes.  Faces can be triangulated by pressing Ctrl+T and converted to quads by pressing Alt+J.  

When you force triangulation you often find Blender evens out any inconsistencies, but you can manually rotate edges using the Edge menu if necessary.  When done convert back to quads and your mesh should look a lot a more even.